A seven flavonoids in hot water extract of immature kumquat (Citrus japonica var. margarita) were identified and quantified (mg/100g fresh fruit): 3',5'-di-C-?-glucopyranosylphloretin (DGPP, 285.9±2.9mg/100g), acacetin 8-C-neohesperidoside (margaritene, 136.2±2.6mg/100g), acacetin 6-C-neohesperidoside (isomargaritene, 119.1±1.8mg/100g), fortunellin (acacetin 7-O-neohesperidoside, 28.5±0.7mg/100g), apigenin 8-C-neohesperidoside (16.9±0.1mg/100g), poncirin (isosakuranetin 7-O-neohesperidoside, 5.1±0.1mg/100g), and rhoifolin (apigenin 7-O-neohesperidoside, 2.0±0.1mg/100g). When immature kumquat was dried at 110 and 130°C for 0.5h, the antioxidant activity, total phenolic content and identified flavonoids increased. The UV absorbance of browning products of immature kumquat dried at 130°C for 1.5h increased dramatically, while the identified flavonoids decreased. Therefore, it was concluded that drying below 130°C for 1.0h, could release phenolic compounds, which resulted in the increasing antioxidant activity. Drying at 130°C for 1.5h, it might be due to the effect of formed browning products.
Hot water extract of immature calamondin peel contains high total phenolic content, which shows significant correlation to DPPH scavenging potency. By heat treatment, the extraction yields of naringin, tangeretin, ferulic acid, p-coumaric acid and gallic acid increased, but the amount of 3',5'-di-C-?-glucopyranosylphloretin (DGPP) decreased drastically. The major soluble phenolic compounds in the nonpolar extract are nobiletin and tangeretin, while DGPP and hesperidin are in the hot water extract. For insoluble phenolic compounds, ferulic acid, p-coumaric acid and sinapic acid are mainly in ester linkage form. After heat treatment, gallic acid and p-coumaric acid are the major increased soluble and insoluble phenolic acids, respectively. This indicates that high temperature heating (150°C) probably produces two major effects: (1) degradation of flavonoids, such as DGPP and hesperidin; (2) destruction of the cell wall structure, leading to an increase in soluble nobiletin, tangeretin and gallic acid, as well as insoluble ferulic and p-coumaric acids.
The antioxidant activity and the flavonoids of mature and immature calamondin (Citrus mitis Blanco) peel were investigated. The hot water extract of immature calamondin peel exhibited the highest oxygen radical absorbance capacity (ORAC), reducing power, and superoxide scavenging effect. 3,5-Di-C-?-glucopyranosylphloretin, naringin, hesperidin, nobiletin, and tangeretin are the five major flavonoids found in hot water extract with the levels of 6888±522, 2333±157, 1350±94, 165±13, and 8±4 mg/100 g dry basis, respectively. The contents of nobiletin and tangeretin increased after ripening. The hot water extract of immature calamondin peel was fractionated using a semi-preparative HPLC. Fraction VI showed the highest ORAC value (28.02±2.73 mmol Trolox equivalents (TE)/g fraction) and two compounds, naringin and hesperidin, were identified as the major active components attributed to the antioxidant activity. Fraction V contained 3,5-di-C-?-glucopyranosylphloretin, which revealed low ORAC value with 7.43 mmol TE/g fraction. However, it might also contribute to antioxidant activity in immature calamondin peel due to its greatest quantity.
There is broad range of applications in the use of tyrosinase inhibitors for suppressing unwanted hyperpigmentation in human skin and enzymic browning in fruits. In searching effective tyrosinase inhibitors from natural products, the components in unripe calamondin (Citrus mitis Blanco) peel were investigated by performing bioassay-directed fractionation and chromatographic separation coupled with tyrosinase inhibition assay. Herein it is reported for the first time that (1) there is a rich content of 3,5-di-C-?-glucopyranosylphloretin in unripe calamondin peel, 3.69±0.44g/100g dry basis, (2) this C-glycosylated flavonoid showed the strongest inhibitory activity against tyrosinase among the components in this fruit, with an IC(50) of 0.87mg/ml, and (3) that unripe calamondin peel is also a rich source of naringin and hesperidin, 1.25% and 0.73% by dry weight, respectively, which also expressed strong tyrosinase inhibitory property.
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